This invention relates to conductive polymers, and more particularly to conductive doped polyanilines and compositions containing them
The use of conductive materials as additives for plastics is of considerable interest since increased conductivity is desirable for many applications, including electrostatic painting and dissipation of static charges. For the latter purpose, especially, a very short static decay time, on the order of 0.02 second, is desirable.
Among the conductive additives which have been employed are conductive carbon black and conductive carbon fiber. At high loadings, however, carbon black tends to accumulate at the surface of a plastic part and subsequently slough off the plastic part. Carbon fiber is expensive and can decrease impact resistance of the part.
It is also possible to coat the surfaces of particulate non-conductive fillers such as mica, glass fibers or glass spheres with a conductive metal or compound thereof such as copper, silver or antimony-doped tin oxide. This, however, can cause problems including high cost, insufficient adhesion to the base resin and loss in conductivity as a result of oxidation of the metal coating.
Many of these problems can be alleviated by employing a conductive polymer as the additive. A particularly advantageous conductive polymer is polyaniline, which may be prepared by electrochemical methods or chemically by the oxidation of aniline in the presence of a mineral acid under relatively mild conditions.
These preparation methods typically afford the polyaniline as a partially oxidized and partially reduced material, with the reduced portion comprising --C.sub.6 H.sub.4 NH-- structural units and the oxidized portion comprising alternating quinone and phenylene moieties separated by trivalent nitrogen atoms. This partially oxidized and partially reduced polyaniline is known as "emeraldine". It can be readily converted to the fully oxidized state, known as "pernigraniline", or to the fully reduced state, known as "leucoemeraldine". It can also be handled in the protonated state, known as "protonated emeraldine".
Polyaniline prepared by known methods, including the above-described chemical method, has, however, various problems which detract from its utility as a conductive polymer. As a single example of such a problem, such polyaniline is often contaminated with by-products such as the virulently carcinogenic benzidine.
PCT application 95/06,685 provides a summary of the state of the art of conductive polyaniline production and processing as of 1994, including the doping of polyaniline with such materials as alkylbenzenesulfonic acids. It also discloses a process in which doped polyaniline is combined with a polymer and a "solvent-plasticizer" to form a processable composition. Such a process requires a first step of preparing the polyaniline, a second step of doping it and a third step of combining the doped material with a polymer and a solvent-plasticizer.
It remains of interest, therefore, to prepare a polyaniline which is not contaminated with harmful by-products and which can be prepared in a minimum of steps and then blended with plastic materials to produce a processable conductive material.